This fixes a build error with older base releases.

This patch gets rid of most of the pv layer, including all remaining C++
code. Also removed is the C++ test/example code for the pv layer. The
interface (pv.h) is not the same, but similar; mostly it is much simpler
and more statically typed.

Instead, we use teh same binary we produce for the test case, and
add an extra switch -t that enables running the sequencer program.

This is so I can first start up the IOC, then switch to telnet console,
and then start the tests. The console output (quite verbose for some of
the tests) will be much faster in this way and the tests accordingly
more realistic.

- fixed a long-standing problem with parallel builds
- work-arounds for various EPICS base versions

William Steele reported and solved this.

This test was provided by Lewis Muir, it fails with seq-2.1.5.

This is now configurable in test/validate/Makefile.